(Supported in part by NSF MCB 9408249 to J. Aist). For the past several years Dr. Aist and his Ph.D student, Satoshi Inoue, have used the laser microsurgery/optical trapping component of the BMIRR extensively in their research program. This year's goals were to determine where the forces are generated that separate the spindle poles during anaphase B in the fungus Nectria haematococca, and to investigate the role of cytoplasmic dynein in mitosis, nuclear migration and nuclear positioning. To achieve the first two goals laser microsurgery was used to sever spindles during anaphase in control cells and in cytoplasmic-dynein mutants. To achieve the latter two goals the optical trap was used to measure the relative forces needed to arrest nuclear migration, and displace nuclei, in controls and mutants. Two papers were published this year on the results of these studies, which could only be conducted at the BMIRR because of its unique DIC-based laser microsurgery sy stem. Inoue, S., B.G. Turgeon, O. C. Yoder and J. R. Aist (1998). Role of fungal dynein in hyphal growth, microtubule organization, spindle pole body motility and nuclear migration. J. Cell Science 111:1555-1566. Inoue, S., O.C. Yoder, B.G. Turgeon and J. R. Aist. 1998. A cytoplasmic dynein required for mitotic aster formation in vivo. J. Cell Science., 111:2607-2614.